1. Field of the Invention
The present invention relates to medical implants. More particularly, the present invention relates to a medical implant with a non-dissolvable amorphous hydroxyapatite/titanium coating.
2. Description of the Related Art
Over the past fifteen years or so, much effort has been made to develop calcium phosphate-based coatings on metallic substrate, such as commercially pure Ti (c.p. Ti), Ti alloys (typically Tixe2x80x946Al-4V), Co-Cr alloys and stainless steel, for the application of a variety of load-bearing medical implants. The most popular coating among the entire calcium phosphate family was hydroxyapatite (HA) due to its chemical stability and osteoconductivity. The most promising metallic system for implant application were found to be Ti and its alloys due to their biocompatibility and mechanical properties.
Important parameters in the long-term behavior of implants coated with HA include at least high coating-substrate bond strength and a low dissolution rate of the coating. In order to improve coating-substrate (usually a metal) bond strength and other properties, a variety of coating techniques have been explored to develop thin (generally less than 10 xcexcm) coatings of HA and other calcium phosphates. U.S. Pat. No. 4,908,030, xe2x80x9cMethod of Manufacturing Synthetic Bone Coated Surgical Implants,xe2x80x9d discloses a method of forming a thin HA coating on an implant using ion beam sputtering. U.S. Pat. No. 5,817,326, xe2x80x9cProcessing of Hydroxylapatite Coatings on Titanium Alloy Bone Prostheses,xe2x80x9d discloses a method in which one or more layers of HA sol-gel are cured to densify on a titanium alloy implant, followed by a non-line-of-sight ion implantation process, in order to strengthen the adhesion of the HA coating to the substrate. U.S. Pat. No. 5,543,019, xe2x80x9cMethod of Coating Medical Devices and Device Coated Thereby,xe2x80x9d discloses a method of forming a thin coating layer on the surface of an implant using a plasma sputtering process. Other methods developed include pulsed laser deposition and magnetron sputtering.
Another approach to improve the bonding capability of HA coating onto metallic substrate has been the deposition of a composite coating, wherein a metallic phase is introduced to serve as either an intermediate layer or a second (continuous or dispersed) phase in HA matrix. For example, Dasarathy et al., in xe2x80x9cHydroxyapatite/metal composite coatings formed by electrocodeposition,xe2x80x9d J. Biomed. Mater. Res., 31, 81-89 (1996), applied an electro-codeposition process to coat a Co/HA composite coating on Ti substrate with a bond strength up to 37 MPa. Using plasma spray technique, Brossa et al., in xe2x80x9cAdhesion properties of plasma sprayed hydroxyapatite coatings for orthopaedic prostheses,xe2x80x9d Bio-Med. Mater. Eng., 3, 127-136 (1993), and Nakashima et al., in xe2x80x9cHydroxyapatite coating on titanium-sprayed titanium implant,xe2x80x9d in Bioceramics 6, P. Ducheyne and D. Christiansen (eds.), Butterworth-Heinemann, Oxford, 1993, pp. 449-453, applied a double-layer comprising an HA layer on top of a porous Ti precoat on a Ti substrate. This double-layered coating was shown to outperform monolithic HA coating in adhesion properties. In German patent xe2x80x9cCoating of implants,xe2x80x9d Gruner, Heiko (Plasmainevent A.-G.) Ger. Offen. DE 3,516,411 (Cl. C23C4/04) Nov. 12, 1986, Gruner teaches a multi-layered coating comprising a Ti precoat, a Ti/HA composite layer and an HA overlayer by plasma deposition. The multi-layer coated implants show fast and stable fusion between the coated implant and the bone. On Tixe2x80x946Alxe2x80x944V substrate Ferraris et al., in xe2x80x9cVacuum plasma spray deposition of titanium particle/glass-ceramic matrix biocomposites,xe2x80x9d J. Am. Ceram. Soc., 79, 1515-1520 (1996),plasma-sprayed a Ti particle-reinforced bioactive glass composite coating, which exhibited a higher bond strength than that of monolithic bioactive glass coating.
Concerning the dissolution rate of HA coating, much effort has been made on the study of immersion behavior of HA coating in simulated body fluid (SBF). This in vitro study is practically important, since it could be some indication of in vivo behavior of the coating, especially when thin coatings are pursued. During immersion tests, unfortunately, many thin HA coatings are extensively dissolved in short periods of time. For example, using an ion beam dynamic mixing technique Ohtsuka et al., in xe2x80x9cFormation of hydroxyapatite coating on pure titanium substrates by ion beam dynamic mixing,xe2x80x9d Surface Coating Technol., 65, 224-230 (1994), have developed an amorphous HA coating. However, this amorphous film dissolved almost completely within 1 day in Hanks"" solution without further annealing. Ong et al., in , xe2x80x9cStructure, solubility and bond strength of thin calcium phosphate coatings produced by ion beam sputter deposition,xe2x80x9d Biomaterials, 13, 249-254 (1992), found that a thin ( less than 1 xcexcm ) amorphous calcium phosphate coating could be deposited by ion beam sputtering. Nevertheless, this film was also dissolved completely within a few hours in saline solution. Wolke et al., xe2x80x9cIn vivo dissolution behavior of various RF magnetron sputter Caxe2x80x94P coatings,xe2x80x9d J. Biomed. Mater. Res., 39, 524-530 (1998), used radio frequency (RF) magnetron-assisted sputtering technique to deposit on titanium substrate thin ( less than 10 xcexcm ) films of amorphous HA. Again, these amorphous coatings were dissolved when implanted subcutaneously into the back of rabbits.
It is therefore an object of the present invention to provide a medical implant coated with a single amorphous hydroxyapatite layer having both high coating-substrate bond strength and a low dissolution rate of the coating.
The present invention achieves the above described object by providing a medical implant with an non-dissolvable amorphous hydroyapatite/titanium coating. The medical implant with a non-dissolvable amorphous hydroyapatite/titanium coating of the present invention comprises: a substrate, and a surface coating deposited onto the substrate from a composite target comprising 10-75 % by volume of titanium and 90-25% by volume of hydroxyapatite, preferably comprising 10-50% by volume of titanium and 90-50 % by volume of hydroxyapatite. The substrate can be comprised of pure titanium or a titanium alloy, for example Tixe2x80x946Alxe2x80x944V. The surface coating can be less than approximately 10 xcexcm thick. The hydroxyapatite component of the composite target can be calcinated. The weight ratio of Ca+P to titanium in the surface coating is in the range of approximately 0.1-3.0, preferably 0.4-3.0.
The present invention further provides a method for producing a medical implant having a non-dissolvable amorphous hydroxyapatite/titanium coating, comprising the steps of: fabricating a substrate; fabricating a composite target comprising 10-75 % by volume of titanium and 90-25 % by volume of hydroxyapatite; and depositing the composite target onto the substrate to form a surface coating thereupon by using an ion sputtering technique, a laser ablation technique, or a physical vapor deposition technique. An additional step of calcinating the hydroxyapatite can be performed before the step of fabricating the composite target.
The medical implant coated with a amorphous hydroyapatite/titanium coating layer of the present invention offers the advantages of both high coating-substrate bond strength and a low dissolution rate of the coating in simulated body fluid, making it suitable for application in dental implants, orthopedic prosthesis, and other types of medical implants.